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In the golden age of the Protheans, Ilos was a verdant world, dotted with the spires and arches of magnificent cities. Even casual observation shows this is no longer the case. Ilos has been devastated by means unknown, its entire surface changed to the color of rust. The atmosphere shows heightened levels of oxygen. Wildfires, presumably ignited by lightning strikes, can be seen burning on the dark side. This indicates that most - if not all - respirating animal life forms have died off. Surface gravity is a comfortable 1.17 standard G's.

Like the ancient human city of Troy, Ilos is a world known only through second-hand sources. References to Ilos have been found at several other Prothean ruins, though direct study of the world is unlikely to occur.

Ilos lies in a remote area of the Terminus Systems only accessible by the legendary Mu Relay. Four thousand years ago, the Mu Relay was knocked out of position by a supernova and lost. Since then, Ilos and its cluster have been inaccessible.

Occasionally, a university will organize an expedition to chart a route to Ilos using conventional FTL drive. These never get beyond the planning stages due to the distance and danger. The journey could take years or decades, passing through the hostile Terminus Systems and dozens of unexplored systems.

According to Tali, the Migrant Fleet expended some effort in searching for Ilos as the quarians once considered it a settlement prospect.

If the Citadel Council did not survive the Battle of the Citadel, a news report in 2185 will mention that the Alliance's Fifth Fleet makes a stop at Ilos during a tour of the Attican Traverse. Ilos is now being studied by salarian researchers, who protest the Alliance's use of dreadnoughts as an intimidation tactic. If the Council survived, however, the news report instead states that the salarian researchers applaud the Alliance's help in keeping them safe: the batarian government has been demanding that the relics from Ilos be made available for them to study.